Most currently used credit cards, bank notes, passports, and security passes can be forged or falsified with little difficulty using modern reproduction methods. Accordingly, numerous attempts have been made to store authenticating information on credit cards, bank notes and the like to increase the difficulty of forgery or falsification. For example, authenticating information may be recorded in the form of magnetic or optical markings which can be read with the aid of machines.
An effective known method for protecting sheet materials against forgery involves use of a diffraction grating with a well defined structure representing the authenticating information. When illuminated, such a diffracting grating produces a particular diffraction pattern, characteristic of the authenticating information. One such diffraction grating mounted on an information carrier such as a credit card or bank note is disclosed in U.S. Pat. No. 4,129,382. The diffraction grating disclosed in this reference comprises an optically transparent body with parallel elongated ridges formed therein.
U.K. Patent Application GB No. 2054 890 A discloses another type of diffraction grating. A reflecting grating for a Moire fringe measuring system is formed from a member of stainless steel having a polished surface. A coating of an optically transparent material is formed on the polished surface. Parts of the coating are removed to form a pattern of parallel ridges and grooves, the bottommost surfaces of the grooves being defined by exposed areas of stainless steel.
Mass manufacture of diffractive type security elements is often accomplished by means of an embossing process. An embossing mold which carries a micro-relief structure is used to impress the micro-relief structure onto a substrate, thereby forming a light-diffracting relief structure which stores authenticating information. Typically, the substrate is a thermoplastic material. The light diffracting structure may be a hologram or a diffraction grating.
Visual or machine identification of the authenticating information is advantageously effected when the diffracting structure reflects rather than transmits incident radiation. To form a reflecting type diffracting structure, the micro-relief structure formed on the thermoplastic substrate is coated with a reflecting metallic film. Typically, a transparent protective layer is then placed over the metallic film. Such a security element is disclosed in CH-PS No. 635-949. A similar security element is disclosed in U.K. patent application GB No. 2093404A.
While useful under many circumstances, the above described reflective/diffractive security element may not always provide adequate security. One reason for this is that the transparent covering layer can be removed, thereby exposing the metallic coated light diffracting structure. Once exposed, this type of light diffracting structure can be reproduced using relatively accessible technology.
It is an object of the present invention to provide an optically diffracting security element for use in authenticating sheet materials such as bank notes and credit cards which overcomes these difficulties and offers a higher degree of security than the security elements heretofore discussed.